sd-netlink: introduce netlink_message_{read,append}_hw_addr

Hardware addresses come in various shapes and sizes, these new functions
and accomapying data structures account for that instead of hard-coding
a hardware address to the 6 bytes of an ethernet MAC.

src/basic/ether-addr-util.c

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 
 #include <errno.h>
+#include <inttypes.h>
 #include <net/ethernet.h>
 #include <stdio.h>
 #include <sys/types.h>
@@ -9,6 +10,20 @@
 #include "macro.h"
 #include "string-util.h"
 
+char* hw_addr_to_string(const hw_addr_data *addr, char buffer[HW_ADDR_TO_STRING_MAX]) {
+        assert(addr);
+        assert(buffer);
+        assert(addr->length <= HW_ADDR_MAX_SIZE);
+
+        for (size_t i = 0; i < addr->length; i++) {
+                sprintf(&buffer[3*i], "%02"PRIx8, addr->addr.bytes[i]);
+                if (i < addr->length - 1)
+                        buffer[3*i + 2] = ':';
+        }
+
+        return buffer;
+}
+
 char* ether_addr_to_string(const struct ether_addr *addr, char buffer[ETHER_ADDR_TO_STRING_MAX]) {
         assert(addr);
         assert(buffer);

src/basic/ether-addr-util.h

@@ -1,11 +1,35 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
+#include <linux/if_infiniband.h>
 #include <net/ethernet.h>
 #include <stdbool.h>
 
 #include "hash-funcs.h"
 
+/* This is MAX_ADDR_LEN as defined in linux/netdevice.h, but net/if_arp.h
+ * defines a macro of the same name with a much lower size. */
+#define HW_ADDR_MAX_SIZE 32
+
+union hw_addr_union {
+        struct ether_addr ether;
+        uint8_t infiniband[INFINIBAND_ALEN];
+        uint8_t bytes[HW_ADDR_MAX_SIZE];
+};
+
+typedef struct hw_addr_data {
+        union hw_addr_union addr;
+        size_t length;
+} hw_addr_data;
+
+#define HW_ADDR_TO_STRING_MAX (3*HW_ADDR_MAX_SIZE)
+char* hw_addr_to_string(const hw_addr_data *addr, char buffer[HW_ADDR_TO_STRING_MAX]);
+
+/* Use only as function argument, never stand-alone! */
+#define HW_ADDR_TO_STR(hw_addr) hw_addr_to_string((hw_addr), (char[HW_ADDR_TO_STRING_MAX]){})
+
+#define HW_ADDR_NULL ((const hw_addr_data){})
+
 #define ETHER_ADDR_FORMAT_STR "%02X%02X%02X%02X%02X%02X"
 #define ETHER_ADDR_FORMAT_VAL(x) (x).ether_addr_octet[0], (x).ether_addr_octet[1], (x).ether_addr_octet[2], (x).ether_addr_octet[3], (x).ether_addr_octet[4], (x).ether_addr_octet[5]